JP6505232B2 - Composite powder for use in lithium ion battery anodes, method of preparing such composite powder, and method of analyzing such composite powder - Google Patents

Description

  The present invention relates to composite powders for use in the anode of lithium ion batteries, methods of preparing such composite powders, and methods of analyzing such composite powders.

  Lithium ion (Li ion) batteries are currently the best performing batteries and are already standard in portable electronic devices. Furthermore, these batteries are already in widespread use in other industries such as automobiles and storage, and are rapidly gaining ground. The advantage that such batteries realize is a combination of good power performance and high energy density.

  A Li-ion battery typically comprises a number of so-called Li-ion cells, which further comprise a positive electrode (cathode), a negative electrode (anode) and a separator which are immersed in the electrolyte. The most frequently used Li-ion cells for portable applications are developed using electrochemically active materials such as lithium cobalt oxide or lithium nickel manganese cobalt oxide for the cathode and natural or man-made graphite for the anode.

  It is known that one of the important limiting factors affecting battery performance, in particular the energy density of the battery, is the active material at the anode. Thus, new electrochemically active materials based on, for example, tin, aluminum and silicon have been studied and developed over the past decades to improve energy density. Such development is mostly based on the principle of alloying the above active material with Li when Li is incorporated during use.

The best candidate is considered to be silicon, because the theoretical capacities of 4200 mAh / g (by weight) or 2200 mAh / cm ³ (by volume) can be obtained, these capacities are g) not only because it is much larger than other candidates.

  Throughout this document, silicon is intended to mean the element Si in the zero-valent state. The term Si is used to denote elemental Si regardless of its oxidation state, whether it is zero-valent or oxidized.

  However, one problem with using a silicon-based electrochemically active material for the anode is that the volume expansion during charging is large, and this volume expansion is due to lithium ions, for example, due to alloying or insertion. When it is completely incorporated into the active material (this process is often referred to as lithiation), it is as high as 300%. The large volumetric expansion of the silicon-based material during the incorporation of Li can induce loading in the silicon, which can later lead to mechanical degradation of the silicon material.

  Repeated mechanical degradation of the silicon electrochemically active material during periodically repeated charging and discharging of the Li-ion battery may shorten the battery life to unacceptable levels.

  In an attempt to reduce the detrimental effects of silicon volume change, these materials reduce the size of silicon materials to submicron or nanosized Si domains, typically with an average size of less than 500 nm, preferably less than 150 nm, Many research studies have shown that viable solutions can be demonstrated by using as an electrochemically active material.

In order to accommodate the volume change, the silicon domain is usually a composite where the silicon domain is usually mixed with a matrix material which is usually a carbon based material but may optionally be a Si based alloy or SiO ₂ Used as particles.

  Furthermore, the negative effect of silicon is that a thick Solid-Electrolyte Interface (SEI) may be formed on the anode. Since SEI is a complex reaction product of electrolyte and lithium, lithium available for the electrochemical reaction is lost, which in turn reduces cycle performance. The cycle performance is capacity loss per charge and discharge cycle. A thick SEI may further increase the battery's electrical resistance, which may limit the achievable charge and discharge rates.

  In theory, it is known from US Pat. No. 6,589,696 and US Patent Application Publication No. 2006/0134516 that the reaction between the anode active material and the electrolyte can be avoided by placing the coating material on the active particles of the anode material. It is done.

  In fact, this was attempted in these documents by mixing particles of anode material with a polyvinyl alcohol (PVA) solution, evaporating the solvent, and pyrolyzing the resulting product to decompose the PVA into carbon. .

  However, this can provide at most a partial and imperfect coating and does not shield the anode material much from the electrolyte.

The reason is probably related to one or more of the following factors:
The amount of PVA was too low to form a complete coating.
In the disclosed process, a significant proportion of the PVA is a little far from the anode active material until the end and can not be used to form a coating.
The carbon yield of PVA decomposition is only 10-20%, so that the carbon layer shrinks very much during formation and the carbon layer is cracked or uncoated during formation An area arises.
-By desorbing 80 to 90% by weight of the decomposition product gas, the decomposition product gas itself generates a channel in the PVA layer being decomposed during conversion to carbon, making the carbon layer porous and protecting The ability is reduced.

Furthermore, it is believed that molecular oxygen in PVA reacts with silicon during pyrolysis to form SiO ₂ , thereby rendering at least a portion of the silicon inert for electrochemical applications.

U.S. Patent No. 6589696 US Patent Application Publication No. 2006/0134516

  Despite advances in the art of the negative electrode and the electrochemically active material contained therein, there remains a need for further superior electrodes with the ability to further optimize the performance of Li-ion batteries. In particular, for most applications, negative electrodes with improved capacity and coulombic efficiency are desirable.

In order to reduce the above and other problems, the present invention is a composite powder for use in lithium ion battery anodes, wherein the particles of the composite powder comprise silicon based domains in a matrix, the matrix comprising A precursor material which is carbon or can be converted to carbon by heat treatment, wherein each silicon based domain is
Free silicon-based domain not embedded or completely embedded in the matrix or any completely embedded silicon-based domain completely surrounded by the matrix,
The proportion of free silicon-based domains is 4% by weight or less of the total amount of metal or oxidized Si in the composite powder, and the silicon-based domains have a d ₅₀ of 200 nm or less and a d ₉₀ of 1000 nm or less The present invention relates to a composite powder having a size distribution based thereon.

  Free silicon-based domains are herein defined as silicon-based domains that are freely accessible from the outside of the composite powder, as they are not blocked or completely blocked by the matrix material.

  By silicon-based domain is meant primarily silicon clusters with individual boundaries with the matrix. The silicon content in such silicon-based domains is usually 80% by weight or more, preferably 90% by weight or more.

  In practice, such silicon-based domains may either be predominantly silicon elemental clusters or individual silicon particles in a matrix made of different materials. The plurality of such silicon particles are silicon powder.

  Composite powders are, in other words, carbon-based composites in which separately produced silicon nanopowders are agglomerated with separately produced carbon and / or carbon precursors acting as a matrix. In this case, the silicon-based domains are formed by the actual individual silicon particles from silicon nanopowders.

  The silicon based domain may have a thin surface layer of silicon oxide.

  Such a composite powder according to the present invention has a very low tendency to form SEI as compared to a conventional composite powder having a silicon-based domain, and therefore has better cycle performance and, along with a large current, It is easy to use.

  While not being bound by theory, the present inventors believe that this is a contact surface that can occur between the electrolyte and the silicon-based domain rather than conventional powders, even though Si is not a significant component in SEI. Is presumed to be related to less.

A further advantage is that the requirements that can be imposed on the water content of the electrolyte are not as severe. This is due to the following reasons. Water in the electrolyte can react with LiPF ₆ in the electrolyte to produce HF. This HF can corrode silicon, leading to the loss of silicon and the formation of Li ₂ SiF _6, which reduces the conductivity of the electrolyte. To avoid this, the water content in the electrolyte is kept very low, often below 50 ppm. However, to obtain this requires expensive raw materials and / or expensive processing equipment.

  With the low free silicon concentration powders of the present invention, this problem is greatly reduced, as a result of which the tight water restriction requirements of the electrolyte can be relaxed and the overall cost is reduced.

  Preferably, the proportion of free silicon-based domains is less than 3% by weight, preferably less than 2% by weight, more preferably less than 1% by weight of the total amount of Si in the metal or oxidized state in the composite powder. As a result, the above advantages are obtained to a higher degree.

  In a preferred embodiment, the matrix is a pitch or a pyrolyzed pitch.

  Such products have been shown to provide excellent performance in batteries.

  Preferably, the composite powder contains less than 3% by weight, more preferably less than 2% by weight, and most preferably less than 1% oxygen.

  The silicon-based domain may have any shape, for example, not only substantially spherical but also whisker-like, rod-like, plate-like, fibrous and needle-like.

  In a preferred embodiment, the proportion of free silicon-based domains is determined by placing a sample of the composite powder in an alkaline solution for a specified period of time and determining the volume of hydrogen generated after the specified period, 2 moles per mole of reacted silicon. The amount of silicon required to generate this amount of hydrogen is calculated based on the generation of hydrogen, and the calculated amount of silicon is the total amount of Si in the metal or oxidation state present in the sample. It is the ratio as determined by dividing by.

  Such calculations can be easily performed by those skilled in the art based on the well-known ideal gas law.

  This specified time is optimally the time required to completely complete the reaction of the nanosilicon powder that is not part of the composite in alkaline solution, but not longer. This depends, of course, on the chosen temperature and the concentration of the alkaline solution. By choosing these conditions, all free silicon is measured, but not completely embedded silicon is measured imprecisely. This may occur due to the diffusion / permeation of the alkaline solution through the matrix if a longer time period or more severe conditions are selected.

  An example of the designated time is 48 hours with a 1.2 g / L KOH solution at a temperature of 45 ° C. These conditions were found to be sufficient to complete the reaction of pure silicon nanopowder, but not longer than necessary.

  Various easy methods are available to measure the amount of gas. A particularly practical method is to use a gas burette.

  The total amount of Si in the metal or oxidation state is often known from the amount of Si-containing material used to prepare the composite or can be determined by standard chemical analysis.

  In a preferred embodiment, the silicon-based domain is a silicon-based particle, which means that it was an individually distinguishable particle that was present separately from the matrix prior to forming the composite. The reason is that it was not formed together with the matrix.

  In another preferred embodiment, the silicon-based particles are preferably free of elements other than Si and O, so they consist of silicon and Si oxide, unless unavoidable impurities are taken into account.

In a further preferred embodiment, the silicon-based domain has a size distribution based on weight with d ₅₀ ≦ 200 nm and d ₉₀ ≦ 1000 nm.

In a further preferred embodiment, the ratio d ₉₀ / d ₅₀ is less than 10, more preferably less than 7.

The d ₅₀ value is defined as the size of the silicon-based domain corresponding to a 50% by weight cumulative undersize domain size distribution. In other words, for example, when _{d 50} of silicon-based domain size is 93 nm, 50% of the total weight of the domains in the sample to be tested is less than 93 nm. Similarly, d ₉₀ is the domain size at which 90% of the total weight of the domain is smaller.

  If the silicon-based domain is or is an individual free particle, such size distribution can be easily determined by laser diffraction of these particles. As is well known to those skilled in the art, special care must be taken to deaggregate the agglomerates in order to ensure the particle size.

  During the synthesis of silicon-based domains, aggregates of silicon-based domains may be formed. In the context of the present invention, an aggregate is a group of domains that are united into one another in a structure, such that the structure can only be partially divided into individual domains, even if the structure is divided. It is understood as a degree of growth.

  The degree of intergrowth of the aggregates can be influenced, for example, by the parameters of the synthesis process that forms the domain, which can coalesce and then grow together to form aggregates during the formation of the aggregates. Thus, an aggregate feature may be that some or all of the domains are destroyed when attempting to divide them into individual constituent domains.

  Briefly, the definition of domains according to the invention also includes aggregates of domains that are fused together so that they can not be separated without the risk of the domain being destroyed.

  Also, domains may agglomerate due to van der Waals forces and other electromagnetic forces acting between the domains to form agglomerates. Conglomerates, in contrast to aggregates, are understood in the context of the present invention to be understood as meaning only loose association of domains that can be easily broken down into constituent domains, and are considered independently domains I can not.

  Alternatively, such size distribution can be determined optically from SEM and / or TEM images by measuring at least 200 silicon-based domains. This method is suitable when silicon-based domains are present in a matrix in which the silicon-based domains can not be separated, but can also be used for silicon-based powders. It should be noted that the domains mean the smallest individual domains that can be determined optically from SEM or TEM images. The size of the silicon-based domain is then determined as the largest measurable linear distance between two points at the perimeter of the domain.

  Such optical methods provide a number-based domain size distribution, which can be easily converted to a weight-based size distribution via well-known mathematical formulas.

  The present invention is further directed to a method of preparing a composite powder, wherein the particles of the composite powder comprise silicon-based domains in a matrix, silicon-based particles, preferably nanosilicon powder, in a molten matrix material, preferably The invention relates to a process comprising a mixing step of mixing without additional solvent, followed by a step of reducing the size of the product obtained by heat treating the mixture obtained and / or the mixture obtained.

  Preferably, the matrix material in the molten state is a molten pitch.

  Preferably, the mixing steps are all carried out with the matrix in the molten state.

  Preferably, the mixing step is carried out in an extruder.

  This method makes it possible to easily produce an excellent composite powder for use in the anode, presumably due to the fact that the matrix material covers the entire surface of the silicon-based domain.

  In a particular variant, the method is a method of preparing a composite powder according to the invention as defined above.

  In both the composite powder according to the invention and the method according to the invention, the matrix is preferably lithium ion conductive and electron conductive or may be lithium ion conductive and electron conductive by thermal decomposition The precursor material is made of

The present invention further provides a method of determining the weight percentage of silicon-based domains not completely embedded in the matrix, in a composite powder having particles of the composite powder containing silicon-based domains in the matrix, the following steps: For methods that include in the following order:
A: placing a fixed amount of composite powder in an alkaline solution for a specified time;
B: a step of determining the volume of hydrogen generated after the above-mentioned designated time;
C: Calculate the amount of silicon required to generate this amount of hydrogen based on the production of 2 moles of hydrogen for each mole of silicon reacted, and calculate the amount of silicon in the sample Dividing by the total amount of metal present in the metal or in the oxidation state.

  The invention further relates to the use of the composite powder according to the invention in a lithium ion battery for limiting or avoiding the formation of SEI.

  The preparation and characterization of the powders according to the invention are described in the following examples and counterexamples.

Analysis method:
Determination of Free Silicon In order to determine the proportion of free silicon-based domains of the product, 0.1 g of product having a known total Si content was placed in a 1.2 g / L aqueous KOH solution at 45 ° C. A gas burette was used to collect the generated gas for 48 hours and measure the volume, but other gas measurement methods may be envisioned.

  A baseline test was also conducted which contained only the KOH solution at the same temperature.

  The volume of gas generated in the baseline test, possibly due to the release of absorbed gas from air, was subtracted from the volume of gas generated from the tested product.

Based on the ideal gas law and the finding that the reaction of silicon and KOH will proceed according to one or both of the following reactions, the volume of gas thus calculated is the mass of silicon reacted: Converted to The following reactions all give an equivalent of 2 moles of hydrogen per mole of silicon.
Si + KOH + 5H ₂ O → KH ₇ SiO ₆ + 2H ₂
Si + ₂ KOH + 2 H ₂ O → K ₂ H ₂ SiO ₄ + 2 H ₂

  The percentage of free silicon based domains was defined as the ratio of the amount of silicon reacted to the total amount of Si in the sample.

Determination of oxygen content The oxygen content of the powders in the examples and counterexamples was determined by the following method using a Leco TC600 oxygen-nitrogen analyzer.

  A sample of powder was placed in a sealed tin capsule and the capsule placed in a nickel basket. The basket was placed in a graphite crucible and heated to above 2000 ° C. with helium as the carrier gas.

The sample thereby melts and oxygen reacts with the graphite in the crucible to become CO or CO ₂ gas. These gases are directed to an infrared measuring cell. Recalculate the observed signal to oxygen content.

Determination of electrochemical performance All composite powders to be tested were sieved using a 45 μm sieve and mixed with carbon black, carbon fibers and sodium carboxymethylcellulose binder in water (2.5% by weight). The ratio used was 90 parts by weight of composite powder / 3 parts by weight of carbon black / 2 parts by weight of carbon fiber and 5 parts by weight of carboxymethylcellulose (CMC).

  The ingredients were mixed in a Pulverisette 7 planetary ball mill in two steps at 500 rpm for 10 minutes.

A copper foil cleaned with ethanol was used as a current collector. A 125 μm thick layer of mixed components was coated on copper foil. The coating was dried at 50 ° C. in vacuum for 45 minutes. A 1.27 cm ² circle was punched out of the dried coated copper foil and used as an electrode in a coin cell using lithium metal as a counter electrode. The electrolyte was 1M LiPF ₆ dissolved in EC / DEC 1/1 + 2 % VC + 10% FEC solvent. All samples were tested on a high precision coin cell tester (Maccor 4000 series).

  The coulombic efficiency of the repeated charge and discharge cycle was determined. The coulombic efficiency of the 9th cycle is reported because it represents the average between the 5th and 100th cycles.

  Those skilled in the art are aware that small changes in coulombic efficiency from cycle to cycle have significant cumulative effects over hundreds or thousands of charge and discharge cycles that the battery is expected to last.

Determination of Size Distribution of Silicon-Based Domain The particle size distribution of the silicon powder of Example 1 and the counterexample was determined by the following method.

  0.5 g of Si powder and 99.50 g of demineralized water were mixed and dispersed using an ultrasonic probe at 225 W for 2 minutes.

  The size distribution was determined on a Malvern Mastersizer 2000, using ultrasound during the measurement, using a refractive index of Si 3.5 and an absorption coefficient of 0.1, ensuring a detection threshold of 5-15%.

Example and Counterexample Starting Materials Nanosilicon powder was prepared as follows:
A micron sized silicon powder was provided as a precursor. A 60 kW radio frequency (RF) inductively coupled plasma (ICP) was applied using an argon plasma. As a result of injecting the precursor into the plasma at a rate of 220 g / h, the temperature (i.e., the reaction zone) was generally above about 2000 K.

In this first process step, the precursor was completely vaporized and then nucleated into nanosilicon powder. A flow of argon was used as quench gas immediately downstream of the reaction zone to reduce the temperature of the gas to below 1600K. Thus, metal nuclei were formed. Finally, the passivation step was carried out at a temperature of 100 ° C. for 5 minutes by adding an N ₂ / O ₂ mixture containing 0.15 mol% of oxygen at 100 L / h.

By adjusting the plasma and argon gas flow rate of both the quenching _{gas, d 50} is 80 _nm, to obtain a Nanokei containing powder with a particle _{d 90} is 521 nm. In this case, using the Ar flow rate of 2.5 Nm ³ / h in the plasma, using Ar flow rate of 10 Nm ³ / h as a quench gas.

Example 1
A blend of 8 grams of the above-described nanosilicon powder and 27 grams of petroleum-based pitch powder was made. It was heated to 450 ° C. under N ₂ and as a result the pitch melted and after a 60 minute waiting period it was mixed for 30 minutes using the dispersion disc.

The suspension of nanosilicon in the pitch thus obtained was cooled to room temperature under N ₂ and ground.

  4.4 g of the milled mixture was mixed with 7 g of graphite on a roller bench for 3 hours, after which the resulting mixture was passed through a mill to break up agglomerates.

  The powder was subjected to a thermal post-treatment as follows: The powder was placed in a quartz crucible in a tubular furnace, heated to 1000 ° C. at a heating rate of 3 ° C./min, maintained at this temperature for 2 hours and then cooled . All this was done under an argon atmosphere.

  The calcined product was milled to form a composite powder and sieved through a 400 mesh sieve.

Example 2
A blend of 500 grams of the above-described nanosilicon powder and 600 grams of petroleum-based pitch powder was made.

  The blend was fed to a Haake process 11 extruder with two axes, which was heated to 400 ° C. while the axis was operating at a rotational speed of 150 rpm. The residence time in the extruder was 30 minutes.

  The resulting extrudate was cooled to less than 50.degree. The resulting extrudate appeared visually to be a homogeneous nanosilicon / pitch composite.

The glass vessel inlet and the extrudate extruder is recovered and isolated from the ambient air by flushing with N _2.

  The cooled extrudates were mixed with the graphite powder in a weight ratio of 22:84 and milled on a roller bench for 2 hours after which the resulting mixture was passed through a mill to break up agglomerates.

  Thermal post-treatment, milling and sieving were carried out as in Example 1.

Counterexample 1
A blend of 1 g of the above-described nanosilicon powder and 3.37 g of pitch and 7 g of graphite was prepared and mixed on a roller bench for 3 hours, after which the resulting mixture was passed through a mill to break up agglomerates.

  The above steps were performed at room temperature to prevent the pitch from melting.

  Thermal post-treatment, milling and sieving were carried out as in Example 1.

Counterexample 2
A blend of 1 g of the above-described nanosilicon powder and 3.37 g of pitch and 7 g of graphite was prepared. To this blend, 10 mL of THF was added and this was mixed on a roller bench for 16 hours.

  This was done at room temperature so that the pitch did not melt.

  The mixture is dried on a rotary evaporator at 75 ° C. and a pressure of 30 mBar, after which the mixture obtained is passed through a mill to break up agglomerates.

  Thermal post-treatment, milling and sieving were carried out as in Example 1.

Counterexample 3
The same procedure as in Counterexample 2 was followed, except that the drying of the mixture in the rotary evaporator was replaced by a spray drying at an exhaust temperature of 110 ° C., using nitrogen as the drying gas.

Results The total Si content of the product obtained from Example 1 and the counterexample was determined to be 9.5 +/- 0.5% by chemical analysis.

  The total Si content of the product obtained from Example 2 was determined by chemical analysis to be 10 +/- 0.5%.

  The free silicon content, oxygen content, and electrochemical performance of all products were measured as described above. The results are reported in Table 1.

  It should be noted that under certain measurement conditions, 0.3% free silicon was the detection limit. This detection limit can be lowered by the person skilled in the art by increasing the sample size and / or by lowering the measurement limit of the generated gas.

  It can be seen from the measured low free silicon content product that a much better coulombic efficiency is obtained, which leads to the better cycling performance of batteries using such powders as anode.

Claims (11)

A composite powder for use in the anode of a lithium ion battery, wherein the particles of said composite powder comprise silicon based domains in a matrix, said matrix being carbon or precursor material which can be converted to carbon by heat treatment Completely embedded silicon-based, wherein the individual silicon-based domains are completely embedded by the free silicon-based domain not embedded or completely embedded in the matrix, or the matrix The ratio of the free silicon-based domain is 4% by weight or less of the total amount of metal or oxidized Si in the composite powder, and the silicon-based domain has a d50 of 200 nm or less. d90 has a size distribution based on weight is 1000nm or less, wherein the silicon-based domain Composite powder down has a surface layer of silicon oxide, the composite powder, graphite also contains the graphite, characterized in that not embedded in the matrix. The proportion of free silicon-based domain, characterized in that it is a 3% by weight less than the total amount of Si metal or oxidation state of the composite powder, the composite powder according to claim 1.   The ratio of the free silicon-based domain is to put a sample of the composite powder into an alkaline solution for a designated time, determine the volume of hydrogen generated after the designated time, and generate 2 moles of hydrogen for each mole of reacted silicon Calculate the amount of silicon needed to generate this amount of hydrogen based on what is being done and divide the calculated amount of silicon by the total amount of Si in the metal or oxidation state present in the sample The composite powder according to claim 1 or 2, characterized in that it is a ratio as determined by   Composite powder according to any of the preceding claims, characterized in that it contains less than 3% by weight of oxygen. It characterized in that it contains a 2 wt% to 25 wt% of S i, composite powder according to claim 1.   The composite powder according to any one of claims 1 to 5, wherein the matrix is a pitch or a pyrolyzed pitch. The silicon-based domain, and wherein the Ru Oh with silicon-based particles, the composite powder according to any one of claims 1 to 6.   Composite powder according to any of the preceding claims, characterized in that it has an average particle size d50 of 1 to 20 micrometers.   Composite powder according to any of the preceding claims, characterized in that at least 97% of the surface area of the silicon based domain is covered by the matrix.   The composite powder according to any one of claims 1 to 9, wherein the silicon-based domain has a weight distribution based on a d50 of 100 nm or less and a d90 of 1000 nm or less. Wherein the ratio of the d50 average size and the silicon-based domain composite powder, characterized in that it is a ten or more, the composite powder according to any one of claims 1 to 10.